In the transmission of a self-clocking digital data stream, it is usually essential to the proper operation of digital apparatus that the clocking signal be correctly extracted. When a derived clock signal is improperly generated from the input self-clocking digital data stream, it is highly desirable to detect such an error as quickly as possible in order to halt the operation of the apparatus and to effect recovery procedures, such as a retransmission of the message. Many techniques have been implemented to detect transmission errors such as parity check or cyclic redundancy check, however, these techniques are more adapted to detection of data errors. Techniques for detecting recovered clock signals errors have been heretofore implemented. Such previous techniques have generally required approximately four bit times or more before an error in the recovered clock can be detected.